During maneuvering, it may be necessary for the wheels on opposite sides of a vehicle to rotate at different rates or even in opposite directions. For example, as a four-wheel vehicle rounds a curve, the outer wheels travel a greater distance and therefore must turn faster than the inner wheels. Maneuvering in tight quarters can cause opposing wheels to turn in opposite directions. This presents no difficulties if the wheels are either driven independently or mounted on a dead axle for independent rotation, however, with a live axle some compensation is necessary to permit the wheels to turn at different speeds.
Differentials or differential gearing have long been utilized for distributing power between the wheels while permitting one wheel to turn faster than the other, as needed on curves. The differentials of the prior art typically include a ring gear driven by a pinion gear mounted on the drive shaft. The ring gear is secured to a differential case or housing for rotation therewith. Each axle includes a coaxial bevel gear which meshes at right angles with pinions mounted on spindles within the differential case. When traveling straight ahead, the differential case is simply driven by the ring gear, and there is no relative motion between the pinion and bevel gears therein. When rounding a curve, however, one wheel must travel relatively faster, and the differential rotation of the axles is compensated for by the pinion gears which permit opposite relative rotation of the bevel gears as the pinion gears are being driven by the differential case, such that faster rotation of one axle and wheel is offset by proportionately slower rotation of the other axle and corresponding wheel.
One of the main disadvantages of conventional differentials has been that all power can be applied to one wheel to the exclusion of the other. That is, if one wheel slips on ice or mud while the other wheel is resting on dry pavement, the differential case and pinion gears therein simply turn around the stationary bevel gear of the axle of the wheel with traction on dry pavement, while the bevel gear for the axle of the wheel without traction simply turns with the pinion gears inside the rotating case about the bevel gear for the other axle.
Another disadvantage of the prior differentials has been that there is no provision for controlling the maximum amount of differentiation between opposite axles. This is usually unnecessary when the vehicle is under control and is being operated within design conditions, however, it can be a significant consideration if the vehicle should spin. In a spin, like those experienced when a driver loses control of his vehicle, such as during auto racing, the wheels on opposite sides of the vehicle tend to rotate in opposite directions, which further contributes to the lack of controllability of the vehicle. It would be far preferable to be able to limit the maximum amount of differentiation between the axles. so that, in the event of a spin, the differential would drag, causing the vehicle to slide in a more predictable, controllable and safe manner.
Various positive or so-called non-slip differentials have been available heretofore, however, the prior differentials have been unnecessarily complex and expensive. One of the most popular non-slip differentials of the prior art operates only in forward gear but not in reverse. Moreover, the prior non-slip differentials only limit slip between the drive shaft and the axles, but not between the axles.
A need has thus arisen for an improved positive differential which effects differentiation between a drive shaft and two driven axles so that neither axle can be driven to the exclusion of the other, but which also limits the maximum amount of differentiation between the axles so that they will not turn uncontrollably in opposite directions should the vehicle spin.